1. Field of the Invention
The present invention relates to a frequency-tunable terahertz light source device, and more particularly, to a frequency-tunable terahertz light source device that can be miniaturized, stably generate a terahertz wave from two different modes and readily change a frequency.
This work was supported by the IT R&D program of MIC/IITA [2006-S-059-02, Development of ASON-based Metro Photonic Cross-Connect Technology].
2. Discussion of Related Art
Technology for fabricating a light source generating a terahertz wave that is an electromagnetic wave between light waves and microwaves has been recently researched.
Terahertz waves travel in a straight line as visible rays, penetrate various materials as electric waves, thus capable of sensing counterfeit bills, narcotics, explosives, biochemical weapons, etc., and checking industrial structures without demolition. Therefore, terahertz waves are expected to be widely used in the fields of general industry, national defense, security, etc., as well as basic science such as physics, chemistry, biology and medical science. Also in the field of information and communication technology, terahertz technology is expected to be extensively used for wireless communication at a data rate of 40 Gbit/s or more, high-speed data processing, and inter-satellite communication.
Thus far, several methods for generating a pulse-shaped terahertz wave and a continuous terahertz wave have been researched, including various techniques such as frequency doubling, a backward-wave oscillator, photomixing, a CO2 pump laser, a quantum cascade laser and a free electron laser.
Among the above mentioned techniques of generating a terahertz wave, photomixing is used for generating a terahertz wave that can be changed, continuously oscillates and has a very small bandwidth.
According to photomixing, two laser beams having different wavelengths are spatially combined in a photoconductive material having carriers having a very short lifetime or in a Unitravelling-Carrier-Photodiode (UTC-PD), thereby generating a terahertz wave corresponding to a wavelength difference between the two laser beams.
When such photomixing is used, a terahertz wave is generated by interference between two laser beams having different wavelengths. Thus, a characteristic of the terahertz wave is determined according to characteristics of the two laser beams and mutual coherence between them.
Therefore, in order to implement a terahertz wave light source that can readily change a generated frequency and stably generate a terahertz wave from two different modes, two laser diodes must emit laser beams that are very stable and coherent with each other and have variable wavelengths. In addition, it is important to monolithically integrate the two laser diodes and implement them in a small size.
However, most techniques that have been hitherto used for photomixing control two longitudinal mode spacings of two high-power solid-state lasers or semiconductor lasers, thereby making a frequency difference between the two modes to be terahertz. Therefore, it is difficult for the techniques to stably generate a terahertz wave, change a generated frequency and be implemented in a small size module.
As an example, a method has been disclosed which inputs two excitation light beams to a waveguide to generate a high-power terahertz wave. However, the method has low efficiency in generating a terahertz wave and has a problem in stability because phase modulation between the frequencies of the two modes is impossible.
As another example, a method has been disclosed which designs a Distributed Feedback (DFB) laser to oscillate two side modes and generates a terahertz wave using a frequency difference between the two side modes. Since the method uses one gain medium, a dynamic range and a frequency are limited.
As yet another example, a method has been disclosed which generates a terahertz wave using a multisection DFB laser device comprising two DFB sections having different grating periods and a phase tuning section. According to the method, the variable wavelength range of the DFB laser is limited to several nanometers, and thus a difference between two modes is no more than several nanometers. Consequently, the frequency variation of the generated terahertz wave does not reach terahertz.
Wavelength-tunable light source devices (wavelength-tunable lasers) are in the limelight as the light source of Wavelength Division Multiplexing (WDM) optical communication systems. To change light of a single wavelength with light of a specific wavelength, external-resonator-type wavelength-tunable light source devices in Littman-Metcalf or Littrow configuration are generally used.
FIG. 1A illustrates the structure of a conventional external-resonator-type wavelength-tunable light source device in Littman-Metcalf configuration.
Referring to FIG. 1A, the external-resonator-type wavelength-tunable light source device in Littman-Metcalf configuration comprises a lens 130 for collimating beams generated from a laser diode 110 having a wide wavelength band, a diffraction grating 150 for diffracting the collimated beams, and a reflection mirror 170 for reflecting the diffracted beams.
When beams are generated from the laser diode 110, they are collimated by the lens 130, and the collimated beams are diffracted toward the reflection mirror 170 by the diffraction grating 150. The angle of the reflection mirror 170 with respect to the diffraction grating 150 is adjusted by a mechanical device (not shown), and thus the reflection mirror 170 reflects only perpendicularly incident light of a specific wavelength among incident wavelengths to the diffraction grating 150. The beam reflected to the diffraction grating 150 is diffracted again by the diffraction grating 150 and returns to the laser diode 110 through the lens 130.
In other words, in the external-resonator-type wavelength-tunable light source device in Littman-Metcalf configuration, the wavelength of a beam returning to the laser diode 110 varies according to the angle of the reflection mirror 170 with respect to the diffraction grating 150.
Meanwhile, an external-resonator-type wavelength-tunable light source device in Littrow configuration has a similar constitution to the external-resonator-type wavelength-tunable light source device in Littman-Metcalf configuration. However, in the external-resonator-type wavelength-tunable light source device in Littrow configuration, the angle of a diffraction grating other than a reflection mirror is adjusted to change a wavelength.
FIG. 1B illustrates the structure of an external-resonator-type wavelength-tunable light source device in Littrow configuration.
Referring to FIG. 1B, when beams are generated from a laser diode 110 in the external-resonator-type wavelength-tunable light source device in Littrow configuration, they are collimated by a lens 130, and a beam having a specific wavelength among the collimated beams is diffracted according to the angle of a diffraction grating 150 and returns to the laser diode 110 through the lens 130. In other words, the wavelength of a beam returning to the laser diode 110 varies according to the angle of the diffraction grating 150 with respect to the lens 130.
As described above, in the conventional external-resonator-type wavelength-tunable light source devices, a reflection mirror or diffraction grating is mechanically rotated to adjust an angle and select a beam of a specific wavelength, and thus must be mechanically and precisely rotated. Therefore, a high precision rotation device for selecting a specific wavelength is necessary, a tunable wavelength range is small, and it is difficult to miniaturize the corresponding module.
As a result, new technology is required for fabricating a light source that has a wide variable wavelength range and a high wavelength change rate, requires no structural movement, and can be readily miniaturized.